Sulphonic fluorinated ionomers

ABSTRACT

Sulphonic fluorinated ionomers crosslinked by radical crosslinking of:  
     A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380-1300 g/eq and comprising: from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene (TFE);  
     from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups —SO 2 F; from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula:  
                 
 
      wherein: m=2-10, preferably 4-8; R 1 , R 2 , R 5 , R 6 , equal to or different from each other, are H or C 1 -C 5  alkyl groups;  
     B) a fluorinated compound as crosslinking radical initiator;  
     C) a fluorinated bis-olefin of structure (I) as crosslinking agent;  
     the radical crosslinking being carried out at a temperature in the range 250° C.-310° C., preferably 260° C.-300° C.

[0001] The present invention relates to sulphonic fluorinated ionomerscrosslinked by radical route suitable for the preparation of membranesfor electrochemical applications, in particular for fuel cells, and asionic exchange resins acting as catalysts.

[0002] Specifically, the invention relates to sulphonic fluorinatedionomers crosslinked by radical route characterized by a high hydrationdegree, both at room temperature and at high temperature (up to 180°C.), without substantially compromising the physical integrity of theobtained membranes, wherefore they are usable also at high temperaturesin the range of 120° C.-180° C.

[0003] It is known in the prior art the use of the class of polymerscalled by the term “ionomers” in electrochemical applications, such forexample in fuel cells, chlorosoda cells, lithium batteries and inreactors in which the ionomer acts as a solid catalyst. Theseapplications implies the contact of the ionomer with a liquid, inparticular water, having affinity with the ionic functional groups ofthe ionomer itself.

[0004] Generally, the larger the amount of ionic groups present in thechain, the better the efficiency of the ionomer application, both interms of capability of ionic exchange in electrochemical applications,and in terms of catalyst activity in catalysis applications. From thispoint of view, an important parameter is the equivalent weight of thisionomer. The lower the equivalent weight, the higher the percentage ofsulphonic groups present in the chain. Therefore, ionomers having a lowequivalent weight are desirable since they give a higher applicationefficiency.

[0005] In electrochemical applications, for example in fuel cells, thereis a direct correlation between the polymer conductivity and the waterretention from the ionomer itself. The ionic conductivity of thepolymer, besides being increased by the greater presence of ionic groupsin the polymer, results increased, within an upper limit, also by thelarger amount of water that the polymer is capable to retain (swellingdegree). However, the excessive affinity of the ionomer with water hasas a consequence the drawback of an excessive polymer swelling, whichassumes a gelatinous state consequently losing its physical integrity.The ionomer becomes therefore completely unusable in all theapplications wherein it is required under a solid form.

[0006] Also in the applications wherein the ionomer is mixed with ordeposited on a support material, suitable to guarantee the shape and thephysical integrity of the final membrane, the ionomer must however showa physical consistency sufficient to prevent the release thereof fromthe support and it must be insoluble in the liquid medium with which itcomes into contact during the use.

[0007] Besides, the ionomeric membrane must be activated before the use,wherefore the chemical transformation of the precursor groups —SO₂F intothe corresponding sulphonic groups —SO₃H is necessary. The membraneactivation is carried out first by contacting it with an alkalineaqueous solution and then with an acid solution. During thistransformation phase, if the ionomer has a high swelling degree, it canpartially or completely dissolve in the reaction medium. At this point,it is impossible to recover the ionomer and separate it from the otherproducts of the transformation reaction.

[0008] In the prior art, to obtain a limited ionomer hydration andsufficient physical integrity, polymers having a high equivalent weight,of the order of 1,000-1,200, are used i.e. having a low concentration ofsulphonic groups, which represent the hydrophilic part of the polymer.Therefore, ionomers having a high equivalent weight absorb a limitedamount of water, which guarantees the polymer insolubility. On the otherhand, having few ionic groups, they have the drawback to give membraneshaving a lower ionic conductivity during the application. An example ofsaid ionomers is represented by the commercial product NAFION®, used infuel cells and having an equivalent weight of the order of 1,000-1,100.The membranes obtained from said ionomers have good mechanicalproperties. However, if these membranes are used at temperatures higherthan 100° C., the interstitial water, which is the carrier of theprotons H⁺ in fuel cells, tends to reduce itself, wherefore the membranetends to dehydrate and the membrane conductivity is drastically reduced.As a consequence, the membranes obtained by NAFION® are not efficientlyusable at temperatures higher than 100° C.

[0009] U.S. Pat. No. 4,940,525 describes sulphonic ionomers having a lowequivalent weight, lower than 725, used to obtain unsupported thickmembranes for fuel cells, only if the hydration product of the polymeris low, lower than 22,000. So low hydration values are indeed necessaryfor maintaining the physical integrity of the ionomer having equivalentweights lower than 725, provided that the equivalent weight is not lowerthan 500 (col. 6, 8-16). Therefore, according to the description of thispatent, it is impossible to obtain sulphonic ionomers of equivalentweight lower than 500 having the property of the insolubility in water.Besides, no mention is made to the behavior of the membranes at hightemperatures, of the order of 120° C.-160° C.

[0010] The need was felt to have available sulphonic fluorinatedionomers such that the obtained membranes have a high hydrationpercentage, both at room temperature and at high temperature (up toabout 180° C.) without substantially compromising the membrane physicalintegrity, wherefore the membranes are usable also at high temperatures,of the order of 120° C.-180° C., in electrochemical applications. Saidmembranes can be used also as ionic exchange resins.

[0011] An object of the present invention are therefore crosslinkedsulphonic fluorinated ionomers obtainable by radical crosslinking of:

[0012] A) crosslinkable sulphonic fluorinated ionomers, havingequivalent weight 380-1300 g/eq, preferably 380-800 g/eq, andcomprising:

[0013] from 48% to 85% by moles of monomeric units deriving fromtetrafluoroethylene (TFE);

[0014] from 15% to 47% by moles of fluorinated monomeric unitscontaining sulphonyl groups —SO₂F;

[0015] from 0.01% to 5% by moles of monomeric units deriving from abis-olefin of formula:

[0016]  wherein: m=2-10, preferably 4-8;

[0017] R₁, R₂, R₅, R₆, equal to or different from each other, are H orC₁-C₅ alkyl groups;

[0018] B) a fluorinated compound as crosslinking radical initiator;

[0019] C) a fluorinated bis-olefin of the above structure (I) ascrosslinking agent;

[0020] the radical crosslinking being carried out at a temperature inthe range 250° C.-310° C., preferably 260° C.-300° C.

[0021] Among the fluorinated monomers containing sulphonyl groups —SO₂Fwe can mention:

[0022] F₂C═CF—O—CF₂—CF₂—SO₂F;

[0023] F₂C═CF—O—[CF₂—CXF—O]_(n), —CF₂—CF₂—SO₂F

[0024] wherein X=Cl,F or CF₃; n′=1-10;

[0025] F₂C═CF—O—CF₂—CF₂—CF₂—SO₂F (vinylsulphonylfluoride);

[0026] F₂C═CF—Ar—SO₂F wherein Ar is an aryl ring.

[0027] Preferably the crosslinkable fluorinated sulphonic ionomers A)comprise:

[0028] from 54% to 71% by moles of monomeric units deriving from TFE;

[0029] from 45% to 28% by moles of monomeric units deriving fromvinylsulphonylfluoride F₂C═CF—O—CF₂—CF₂—SO₂F;

[0030] an amount higher than 0.4% by moles up to 3% by moles, morepreferably from 1% to 2.5% by moles of monomeric units deriving from thebis-olefin of formula (I).

[0031] As regards the radical initiators B) used in the radicalcrosslinking of the present invention, they are selected from:

[0032] (d)-branched perfluoroalkanes of formula:

C _(a) F _(2a+2) wherein a=5-15, preferably 7-11;

[0033] (e)-halogenated compounds of formula:

ClO₂S(CF₂)_(n)SO₂Cl wherein n=4-10;

[0034] (f)-peroxidic perfluoropolyether compounds having oxidizing powerin the range 0.8-6, preferably 1-3.5, of structure

T—O—(R_(f))—(O)_(c)—T′

[0035] wherein:

T,T′=—CF₃,—CF₂CF₃, —CF₂CF₂CF₃

[0036] c is an integer such as to give the above oxidizing power;

[0037] R_(f) perfluoropolyether chain having a number average molecularweight in the range 1,000 and 30,000, preferably 4,000-20,000,comprising one or more of the following units:

—(CF₂O)—, —(CF₂CF₂O)—, —(CF₂CF₂CF₂O)—, —(C₃F₆O)—

[0038] Preferably the perfluoropolyether chain R_(f) is selected fromthe following structures:

—(CF₂O)_(a′)(CF₂CF₂O)_(b′—)

[0039] with b′/a′ in the range 0.1-40, preferably 0.5-20, a′ and b′being integers such as to give the above molecular weight;

—(CF₂O)_(c′)(C₃F₆O)_(d′—)

[0040] with c′/d′ in the range 0.01-5, c′ and d′ being integers such togive the above molecular weight.

[0041] As oxidizing power it is meant the amount in grams of peroxidicoxygen per 100 grams of polymer.

[0042] As radical initiator, a peroxidic compound of formula (f) withthe above oxidizing power is more preferably used. The preparation ofthe peroxidic initiators (f) can be carried out according to theprocesses described in U.S. Pat. Nos. 3,847,978 and 5,488,181.

[0043] It has been found by the Applicant that both in the case of toolow oxidizing power and in the case of too high oxidizing power, thecrosslinking of the ionomeric chains is poor. The Applicant has foundthat using as initiator the compound of formula (f) having an oxidizingpower within the above values, a very good level of crosslinking isobtained.

[0044] When a radical initiatior of formula (f) is used for thecrosslinking, it is fed in a concentration in the range 2%-10% byweight, preferably 3-6% by weight with respect to the weight of thesulphonic ionomer.

[0045] As regards the crosslinking agent bis-olefin of formula (I), itis fed in a concentration in the range 3%-25% by weight, preferably 4-8%with respect to the weight of the sulphonic ionomer.

[0046] According to a not binding theory, the Applicant keeps that inthe crosslinking initial phase, the radical initiator has the functionto remove a proton from the bis-olefin of formula (I) present in thechain of the sulphonic ionomer. The so formed radical can attack thedouble bond of the bis-olefin c) introduced as crosslinking agent andgenerate the first part of the reticule. The repetition of this H*extraction event on another ionomeric chain allows the reticuleformation.

[0047] A further object of the invention are the supported orself-supported membranes, and their process of preparation by using thecrosslinked sulphonic fluorinated ionomers of the present invention.

[0048] For supported membranes, foamed PTFE, preferably bistretched, canbe used as support, having a suitable dimension in connection with theelectrochemical cell, having a thickness comprised between 10 μm and 50μm. For the preparation of supported membranes, first a solution isprepared formed by:

[0049] A) a solution of the sulphonic ionomer in a fluorinated solventselected from: hexafluoroxylene, perfluorohexane, perfluorooctane,perfluorobenzene, perfluoropolyether solvents, fluoroether solvents.

[0050] B) a radical initiator of formula (d), (e) or (f);

[0051] C) the bis-olefin of formula (I) as crosslinking agent.

[0052] The porous support of foamed PTFE is dipped in the solutionformed by A)+B)+C) for a time comprised between 10 seconds and 1 minute;the impregnation is repeated more times until an impregnated membrane isobtained. Subsequently the ionomeric solution excess is removed from thesupport, for example by a roller system. The supported membrane obtainedby impregnation is then dried at 25° C. for about 1 hour, to remove thefluorinated solvent.

[0053] At this point, the membrane is crosslinked at the above mentionedtemperature, for a reaction time equal to 6 half-lives of the usedinitiator, i.e. until a decomposition of about 99% of the initiator.When a radical initiator of formula (f) is used, the crosslinkingaverage time generally ranges from about 10 seconds to 3 minutes.

[0054] Alternatively, the preparation of the supported membrane can becarried out by casting. In the case of thick membrane (self-supported),the membrane can be obtained by casting or by press. In the latter casea film of the solution A)+B)+C) is placed between two metal platescontaining a metal frame, which confers to the resulting membrane thedesired thickness. Then, the plate/frame/film system is put in a pressoven applying a weight of about 500-2,000 kg, preferably 750-1,500 Kg.

[0055] The supported or thick membranes, obtained from the sulphonicionomers crosslinked by the method of the present invention have athickness ranging from 10 μm to 300 μm depending on the used preparationmethod. The membrane appears transparent, in some cases lightbrown-colored. The morphological analysis at the microscope of theobtained membrane shows a substantially uniform, smooth and completelyfree from holes surface.

[0056] The obtained membrane, when it appears light brown-colored, canbe decolorated. Decoloration can be carried out by dipping the membranein the —SO₂F form into an aqueous solution containing H₂O₂ at 9%(weight/volume) and HNO₃ at 10% (weight/volume) at 50° C. for about 4-6hours. A colorless and transparent membrane is obtained with a maximumweight loss lower than 1% by weight (color loss higher than 95% by UVanalysis). The decoloration treatment can be carried out both before andafter the membrane activation treatment described hereunder.

[0057] The membrane is then subjected to the activation treatment fortransforming the sulphonyl groups —SO₂F into sulphonic groups —SO₃H. Theactivation implies 2 steps:

[0058] salification for transforming the —SO₂F form into the —SO₃ ⁻form;

[0059] acidification for transforming the —SO₃ ⁻ form into the —SO₃Hform.

[0060] Salification is carried out by dipping the membrane obtainedafter the crosslinking reaction in a basic aqueous solution of KOH or ofNaOH at a temperature such as to have an almost total conversion intothe —SO₃ ⁻ groups.

[0061] It has been found by the Applicant that, for equivalent weightslower than 800, a temperature in the range 5° C.-40° C., for a timecomprised between 4 and 40 hours, can be used. The 97% of the conversiontakes place in the first 20-30 minutes of reaction; further 5 hours arenecessary for obtaining the required conversion of 99.9% (upper limit ofinstrumental detection). The weight loss due to the dissolving of thesalified membrane in water is lower than 20%. Temperatures higher than40° C., shorten the conversion times, but they remarkably increase theweight loss of the membrane (for example operating at 70° C., there is aweight loss equal to 90-95%). Therefore, the crosslinked membranesobtained from the sulphonic fluorinated ionomers having a low equivalentweight of the invention show the additional advantage to be salified atroom temperature.

[0062] At the end of the salification, the membrane is dipped into adistilled water bath at 25° C. for washing the residual base.

[0063] The acidification is carried out by dipping the salified membranein an aqueous solution containing the 20% by weight of HCl at 25° C. for5 hours. The conversion is equal to 99.9% 20 with a weight loss of themembrane in the —SO₃H form lower than 1%. The resulting crosslinked,optionally decolorated, membrane in the —SO₃H form, is suitable to beused in applications of electrochemical type, for example of fuel celltype or in catalysis applications as ionic exchange resin.

[0064] The crosslinked membranes of the invention after activation, i.e.in the —SO₃H form, show the following propeties:

[0065] amount of gels higher than 85%, generally higher than 90%;

[0066] after drying and hydration in water at 100° C. for 30 minutes,the membrane substantially remains integer without dissolving in water.

[0067] As “gel” is meant the insoluble part of the polymer and the % byweight of gels is correlated with the crosslinking degree of the ionomeritself. Indicatively, the higher the % by weight of gels, the lower theamount of uncrosslinked ionomer.

[0068] Tests carried out by the Applicant (see the Examples) show thatthe obtained membranes, notwithstanding the high hydration percentage,have a good physical integrity and they are substantially insoluble inwater both at low and at high temperature (up to about 180° C.). Inparticular, the T_(r) values (interstitial water release temperature)obtained by thermogravimetric analysis TGA, result surprisingly high.

[0069] The interstitial water is the carrier of the H⁺ protons in fuelcells, wherefore the invention membranes, having a T_(r) in the range160° C.-180° C., are capable to mantain a good conductivity also at veryhigh temperatures, for example of the order of about 160° C. This issurprising and unexpected since the membranes of sulphonic ionomersknown in the prior art are not usable in an efficient way attemperatures higher than 100° C.

[0070] Besides in the preparation of membranes for fuel cells, thesulphonic ionomers of the present invention can successfully be used inthe preparation of ionic exchange resins for carrying outchemico-physical separations and as acid catalyst for chemicalreactions. The crosslinked ionomer of the invention, when used as anacid catalyst, appears extremely effective since it allows to obtainhigh yields in short times.

[0071] As regards the preparation of crosslinkable sulphonic fluorinatedionomers, it can be carried out by polymerization in aqueous emulsionaccording to well known methods of the prior art, in the presence ofradical initiators (for example, alkaline or ammonium persulphates,perphosphates, perborates or percarbonates), optionally in combinationwith ferrous, cupric or silver salts, or other easily oxidizable metals.In the reaction medium also surfactants of various type are usuallypresent, among which the fluorinated surfactants of formula:

R_(f)-X⁻M⁺

[0072] are particularly preferred, wherein R_(f) is a C₅-C₁₆(per)fluoroalkyl chain or a (per)fluoropolyoxyalkylene chain, X⁻ is—COO⁻ or —SO₃ ⁻, M⁺ is selected from: H⁺, NH₄ ⁺, alkaline metal ion.Among the most commonly used we remember: ammonium perfluorooctanoate,(per) fluoropolyoxyalkylenes ended with one or more carboxylic groups,etc.

[0073] When the polymerization is over, the ionomer is isolated byconventional methods, such as coagulation by addition of electrolytes orby cooling.

[0074] Alternatively, the polymerization reaction can be carried out inbulk or in suspension, in an organic liquid wherein a suitable radicalinitiator is present, according to well known techniques.

[0075] The polymerization reaction is generally carried out attemperatures in the range 25° C.-150° C., under pressure up to 10 MPa.

[0076] The preparation of the sulphonic fluorinated ionomers of theinvention is preferably carried out in aqueous emulsion in the presenceof an emulsion, dispersion or microemulsion ofperfluoropolyoxyalkylenes, according to U.S. Pat. No. 4,789,717 and U.S.Pat. No. 4,864,006.

[0077] The present invention will be now better illustrated by thefollowing embodiment Examples, which have a merely indicative purposebut not limitative of the scope of the invention itself.

EXAMPLES Characterization Percentage of Gels

[0078] The “gel” term is defined as the insoluble part of a polymer inthe same solvent wherein it was soluble before the crosslinking. The %of gels is therefore correlated with the crosslinking degree of thepolymer itself.

[0079] The gel % is evaluated before and after the crosslinking by thefollowing procedure:

[0080] the ionomer is solubilized in perfluorohexane with anionomer/solvent concentration of about 25% (weight/volume);

[0081] the solution is left under stirring at 40° C. for 24 hours; atthe end the % by weight of ionomer which is not dissolved is evaluated.

Percentage of Hydration

[0082] After drying (1 hour at 50° C.), the membrane is weighed andsubsequently hydrated in distilled water at 100° C. for 30 minutes; thenit is extracted from water, dried on the surface and weighed again.

[0083] The hydration percentage H% of the membrane is evaluatedaccording to the following formula:

H%=100×(hydrated membrane weight−dried membrane weight/dried membraneweight)

Percentage of Extractable Substances

[0084] The dried membrane is first weighed and then put in a solution ofethanol/water 40/60 by weight at 50° C. for 22 hours. Subsequently thesolution is filtered on filter Whatman 541. The filtered product isdried at 80° C. and the dry residue is weighed.

[0085] The percentage of extractable substances E% is calculatedaccording to the following formula:

E%=100×(dry residue weight/membrane initial weight)

[0086] The lower the percentage of extractable E%, the higher thechemico-physical resistance of the obtained membrane.

Release Temperature of the Interstitial Water

[0087] The release temperature T_(r) of the interstitial water isevaluated by thermogravimetric analysis (TGA).

[0088] An amount of about 10 mg of the membrane, hydrated in distilledwater at 100° C. for 30 minutes, is analyzed in a thermogravimetricanalyser Perkin Elmer model TGA7. After having been maintained in N₂flow at room temperature, the sample is heated with a temperaturegradient of 10° C./min up to 80° C., temperature at which it ismaintained for 5 minutes. Subsequently the sample is cooled at the samerate until room temperature, at which it is maintained for 10 minutes.Then the thermogravimetric analysis starts by heating the sample at arate of 10° C./min.

[0089] The TGA curve derivative allows to go back to the value of therelease temperature T_(r) in correspondence of the second minimum of thederivative of the TGA curve. The higher the temperature T_(r), thegreater the capability of the membrane to retain water and consequentlyto lead also to high temperatures.

EXAMPLE 1

[0090] In a 2 liter autoclave, the following reactants are introduced:

[0091] 700 ml of demineralized water;

[0092] 45 ml of the monomer of formula CF₂═CF—O—CF₂CF₂—SO₂F;

[0093] 29 g of a microemulsion of perfluoropolyoxyalkylenes previouslyobtained by mixing:

[0094] 11.6 g of a perfluoropolyoxyalkylene having a potassium salifiedacid end group of formula:

ClCF₂O(CF₂—CF(CF₃)O)_(n)(CF₂O)_(m)CF₂COOK

[0095]  wherein n/m=10 having number average molecular weight 527;

[0096] 5.8 g of a perfluoropolyether oil Galden® D02 of formula

CF₃O(CF₂—CF(CF₃)O)_(n)(CF₂O)_(m)CF₃

[0097]  wherein n/m=20 having average molecular weight 450

[0098] 11.6 g of water;

[0099] 240 g of a bis-olefin of formula CH₂═CH—(CF₂)₆—CH═CH₂ fed in aGalden® D02 solvent obtaining a solution at 30% by volume of bis-olefin.

[0100] The autoclave, kept under stirring at 700 rpm, has been heated upto 50° C. 400 ml of an aqueous solution having a concentration of 20 g/lof potassium persulphate (KPS) are then fed into the autoclave. Thepressure is brought to 3 absolute bar introducing TFE. The reactionstarts after 3 min. The pressure is maintained at 3 absolute bar byfeeding TFE.

[0101] The total mass of TFE fed to the reactor is equal to 760 g. Thetotal mass of sulphonyl monomer CF₂═CF—O—CF₂CF₂—SO₂F fed to the reactoris equal to 2,126 g.

[0102] The reaction is stopped after 280 min from the starting,lessening the stirring, cooling the reactor and venting the TFE. Theproduced latex has a solid content of 25% by weight. The latex iscoagulated by freezing, the polymer is separated from the mother liquorsand dried at 100° C. for 8 h at room pressure. The composition by molesof the copolymer determined by NMR results to be:

62.9% TFE, 35.1% sulphonic monomer, 2% bis-olefin. The equivalent weightresults of 480 g/eq.

[0103] On a sample of sulphonic ionomer obtained from thepolymerization, the gel % is evaluated according to the above procedure.A value of gels equal to 10% is obtained and the ionomer almostcompletely dissolves in solution.

Crosslinking

[0104] 17.64 g of ionomer obtained by the above described polymerizationare dissolved in 82.36 g of a perfluoropolyether solvent of formulaCF₃CF₂O—(C₃F₆O)₃—CF₂H obtaining an ionomeric solution having an ionomerconcentration equal to 17.64% by weight.

[0105] To 100 g of the ionomeric solution:

[0106] 5% by weight referred to the ionomer of the bis-olefin of formula

CH₂═CH—(CF₂)₆—CH═CH₂;

[0107] 4% by weight of an initiator having the formula:

T—O—(R_(f))—(O)_(c)—T′

[0108]  are added

[0109] wherein:

T,T′═—CF₃

[0110] c is an integer such to give an oxidizing power equal to 2;R_(f)=perfluoropolyether chain having a number average molecular weightin the range 6,500-7,500, and structure:

—(CF₂O)_(a′)(CF₂CF₂O)_(b′—)

[0111] with b′/a′ in the range 0.5 and 20, a′ and b′ being integers suchas to give the above indicated molecular weight.

[0112] A mother solution containing sulphonic ionomer, bis-olefin andradical initiator is obtained.

[0113] 85% of the solvent is evaporated under light nitrogen flow. Theremaining 15% of the solvent is evaporated, leaving the mother solutionunder ventilation of an inert gas. An homogeneous film formed byionomer, bis-olefin and radical initiator is thus obtained.

[0114] Crosslinking of the ionomeric film is carried out a temperatureof 270° C. The reaction time is equal to 35 seconds. After crosslinkingthe percentage by weight of gels on the basis of the above procedure isevaluated. A gel value equal to 98% is obtained. The ionomer can thenface the subsequent activation reaction for transforming the sulphonylgroups —SO₂F into sulphonic groups —SO₃H without substantial materialloss.

EXAMPLE 2

[0115] Example 1 is repeated with the difference that the used amount ofbis-olefin of formula CH₂═CH—(CF₂)₆—CH═CH₂ is equal to 15% by weightwith respect to the sulphonic ionomer. A mother solution containingsulphonic ionomer, bis-olefin and radical initiator is obtained.

[0116] 85% of the solvent is evaporated under light nitrogen flow. Theremaining 15% of the solvent is evaporated by leaving the mothersolution under ventilation of an inert gas. An homogeneous film formedby ionomer, bis-olefin and radical initiator is thus obtained.

[0117] Crosslinking of the ionomeric film is carried out a temperatureof 270° C. The reaction time is equal to 35 seconds. After crosslinkinga gel value equal to 96% is obtained.

EXAMPLE 3

[0118] Example 1 is repeated but using the initiator of Example 1 in anamount equal to 3% by weight with respect to the ionomer. Thecrosslinking procedure of Example 1 is repeated. After crosslinkingaccording to the method of the present invention, a gel value equal to90% is obtained.

EXAMPLE 4

[0119] Example 1 is repeated, using an initiator having the samestructure of that of Example 1, but with an oxidizing power equal to 3.5

[0120] The crosslinking temperature is equal to 290° C.

[0121] After crosslinking according to the method of the presentinvention, a gel value equal to 86% is obtained.

EXAMPLE 5 (comparative)

[0122] Example 1 is repeated, but carrying out the crosslinking at thetemperature of 210° C.

[0123] After a reaction time equal to 30 minutes the percentage of gelsis evaluated. A gel value equal to 10% is obtained.

[0124] This Example shows that at the temperature of 210° C. thecrosslinking reaction of the invention does not take place.

EXAMPLE 6 (comparative)

[0125] Example 1 is repeated with the difference that the used amount ofbis-olefin of formula CH₂═CH—(CF₂)₆—CH═CH₂ is equal to 2% by weight withrespect to the sulphonic ionomer.

[0126] The crosslinking procedure of Example 1 is repeated.

[0127] At the end of the crosslinking the gel % results equal to 60%.This Example shows that when the % by weight of crosslinking agent(bis-olefin) is lower than the defined range, the crosslinking reactionof the invention only partially takes place.

EXAMPLE 7 (comparative)

[0128] Example 1 is repeated, using an initiator having the samestructure of that of Example 1, but with an oxidizing power equal to0.4, in an amount of 10% by weight with respect to the ionomer.

[0129] The crosslinking procedure of Example 1 is repeated.

[0130] The gel % results equal to 10%. This Example shows that when theoxidizing power of the initiator is lower than 0.8, the crosslinkingreaction of the invention does not take place.

EXAMPLE 8 Activation

[0131] The film obtained by the crosslinking carried out in Example 1 issubjected to a salification treatment for 6 hours at 25° C. in a KOHaqueous solution at 10% by weight. The 97% of the conversion takes placein the first 20-30 minutes of reaction; further 5 hours are insteadnecessary for obtaining the required conversion of 99.9% (upper limit ofinstrumental detection). The weight loss due to the dissolving of thesalified membrane in water is equal to 14%. At the end of thesalification, the membrane is dipped into a distilled water bath at 25°C. for washing the residual KOH.

[0132] The acidification is carried out by placing the salified membranein an aqueous solution containing the 20% by weight of HCl at 25° C. for5 hours. The conversion is equal to 99.9% with a weight loss of themembrane in the —SO₃H form lower than 1%.

[0133] The previous treatment of salification and acidification involvesthe complete transformation, in the limits detectable at the FTIRanalysis, of the —SO₂F groups into sulphonic groups —SO₃H.

[0134] This Example shows that the crosslinked film according to themethod of the present invention can advantageously be salified at roomtemperature.

EXAMPLE 9

[0135] A membrane is prepared by using as a support, bistretched foamedPTFE having thickness of 35 μm and porosity equal to 0.2 μm.

[0136] 17.64 g of ionomer obtained by the polymerization of Example 1are dissolved in 82.36 g of a perfluoropolyether solvent of formulaCF₃CF₂O—(C₃F₆O)₃—CF₂H obtaining an ionomeric solution having an ionomerconcentration equal to 17.64% by weight.

[0137] To 100 g of the ionomeric solution:

[0138] 5% by weight referred to the ionomer of the bis-olefin of Example1;

[0139] 4% by weight of the radical initiator of Example 1; are added.

[0140] A mother solution containing sulphonic ionomer, bis-olefin andradical initiator is obtained. The porous support of foamed PTFE isdipped into this solution for 10 seconds; the impregnation is repeated 3times, until a 100% impregnated membrane, completely transparent, havingthickness of 110 μm, is obtained. Subsequently the ionomeric solution inexcess is removed from the support by a roller system. The supportedmembrane obtained from the impregnation is then dried at 50° C. forabout 1 hour, to eliminate the fluorinated solvent.

[0141] At this point, the membrane is crosslinked at the temperature of270° C. for a reaction time equal to 35 seconds. Then the membrane isactivated in the —SO₃H form following the procedure described in Example8. The weight loss % during the activation is equal to 12.6%.

[0142] A percentage of extractable substances E% equal to 2% isdetermined.

[0143] The percentage of hydration H% is equal to 40%.

[0144] The release temperature T_(r) of the interstitial water is 175°C.

EXAMPLE 10

[0145] A membrane is prepared following the same impregnation procedureof Example 9, but using as a support monostretched foamed PTFE havingthickness of 60 μm and porosity equal to 0.45 μm.

[0146] As solvent for the ionomeric solution CH₃OC₄F₉ (HFE 7100) isused. Crosslinking takes place at the temperature of 270° C. for areaction time equal to 35 seconds. After crosslinking, the membrane isactivated in the —SO₃H form following the procedure of Example 8.

[0147] The so obtained ionomeric membrane, having thickness of 65 μm, isdried for 1 h at 50° C.

[0148] The percentage of extractable substances E% is 20%.

[0149] The percentage of hydration H% is equal to 40%.

[0150] The release temperature T_(r) of the interstitial water is 168°C.

EXAMPLE 11

[0151] A non supported thick membrane is prepared by casting fromsolvent, using the same perfluoropolyether solvent of Example 9.Crosslinking takes place at the temperature of 270° C. for a reactiontime equal to 35 seconds.

[0152] After crosslinking, the membrane is activated in the —SO₃H formfollowing the procedure of Example 8. The weight loss % during theactivation is equal to 12.6%.

[0153] The so obtained ionomeric membrane, having thickness of 250 μm,is dried for 1 h at 50° C.

[0154] The percentage of extractable substances E% is 21%.

[0155] The percentage of hydration H% is equal to 55%.

[0156] The release temperature T_(r) of the interstitial water is 161°C.

EXAMPLE 12

[0157] A supported membrane is prepared using as a support bi-stretchedfoamed PTFE having thickness of 35 μm and porosity equal to 0.2 μm. Theprocedure by casting is used, using the same perfluoropolyether solventof Example 9.

[0158] Crosslinking takes place at the temperature of 270° C. for areaction time equal to 35 seconds. After crosslinking, the membrane isactivated in the —SO₃H form following the procedure of Example 8.

[0159] The so obtained ionomeric membrane, having thickness of 65 μm, isdried for 1 h at 50° C.

[0160] The percentage of extractable substances E% is 11%.

[0161] The percentage of hydration H% is equal to 92%.

[0162] The release temperature T_(r) of the interstitial water is 164°C.

EXAMPLE 13

[0163] In this Example the membrane obtained in Example 9 is used asacid catalyst in the Friedel-Craft acylation reaction. The Friedel-Craftreaction has been used as a test for evaluating the catalysis efficiencyof the sulphonic groups present in the membrane.

[0164] The 3% by weight of the membrane in the —SO₃H form of Example 9is added to an equimolar mixture of the reactants anisole and aceticanhydride with respect to the anisole.

[0165] The reaction formation of the reaction product4-methoxyacetophenone, carried out at a temperature of 22° C., reaches aconversion of 95% in 25 minutes and it is completed with a conversion of100% in 37 minutes.

[0166] This Example shows that the crosslinked sulphonic fluorinatedionomers by radical route of the invention can advantageously be used asacid catalyst for chemical reactions. TABLE 1 Cross- Initi- Initi-Temper- linking ator ator ature Reaction Gels agent (%) (%) PO (° C.)time (%) Example 1 5 4 2.0 270 35 seconds 98 Example 2 15  4 2.0 270 35seconds 96 Example 3 5 3 2.0 270 35 seconds 90 Example 4 5 4 3.5 290 35seconds 86 Example 5 5 4 2.0 210 30 minutes 10 Comp. Example 6 2 4 2.0270 35 seconds 60 Comp. Example 7 5 15  0.4 270 35 seconds 10 Comp.

[0167] TABLE 2 Membrane Extractable thickness substances Hydration TrSupport Methodology (μm) (%) (%) (° C.) Example 9 bistretchedimpregnation 110  2 40 175 foamed PTFE Example 10 monostretchedimpregnation  65 20 40 168 foamed PTFE Example 11 — casting 250 21 55161 Example 12 bistretched casting  65 11 92 164 foamed PTFE

1. Sulphonic fluorinated ionomers crosslinked by radical crosslinkingof: A) crosslinkable sulphonic fluorinated ionomers, having equivalentweight 380-1300 g/eq, preferably 380-800 g/eq, and comprising: from 48%to 85% by moles of monomeric units deriving from tetrafluoroethylene(TFE); from 15% to 47% by moles of fluorinated monomeric unitscontaining sulphonyl groups —SO₂F; from 0.01% to 5% by moles ofmonomeric units deriving from a bis-olefin of formula:

 wherein: m=2-10, preferably 4-8; R₁, R₂, R₅, R₆, equal to or differentfrom each other, are H or C₁-C₅ alkyl groups; B) a fluorinated compoundas crosslinking radical initiator; C) a fluorinated bis-olefin ofstructure (I) as crosslinking agent; the radical crosslinking beingcarried out at a temperature in the range 250° C.-310° C., preferably260° C.-300° C.
 2. Sulphonic fluorinated ionomers crosslinked accordingto claim 1, wherein the fluorinated monomers containing sulphonyl groups—SO₂F are selected from: F₂C═CF—O—CF₂—CF₂—SO₂F;F₂C═CF—O—[CF₂—CXF—O]_(n)—CF₂—CF₂—SO₂F wherein X=Cl, F or CF₃; n′=1-10;F₂C═CF—O—CF₂—CF₂—CF₂—SO₂F (vinylsulphonylfluoride); F₂C═CF—Ar—SO₂Fwherein Ar is an aryl ring.
 3. Sulphonic fluorinated ionomerscrosslinked according to claims 1 and 2, wherein the crosslinkablesulphonic fluorinated ionomers A) comprise: from 54% to 71% by moles ofmonomeric units deriving from TFE; from 45% to 28% by moles of monomericunits deriving from vinylsulphonylfluoride F₂C═CF—O—CF₂—CF₂—SO₂F; anamount higher than 0.4% by moles up to 3% by moles, more preferably from1% to 2.5% by moles of monomeric units deriving from the bis-olefin offormula (I).
 4. Sulphonic fluorinated ionomers crosslinked according toclaims 1-3, wherein the radical initiators B) are selected from:(d)-branched perfluoroalkanes of formula: C_(a)F_(2a+2) wherein a=5-15,preferably 7-11; (e)-halogenated compounds of formula:ClO₂S(CF₂)_(n)SO₂Cl wherein n=4-10; (f)-peroxidic perfluoropolyethercompounds having oxidizing power in the range 0.8-6, preferably 1-3.5,of structure T—O—(R_(f))—(O)_(c)—T′ wherein:T,T′=—CF₃,—CF₂CF₃,—CF₂CF₂CF₃ c is an integer such as to give the aboveoxidizing power; R_(f) perfluoropolyether chain having a number averagemolecular weight in the range 1,000 and 30,000, preferably 4,000-20,000,comprising one or more of the following units: —(CF₂O)—, —(CF₂CF₂O)—,—(CF₂CF₂CF₂O)—, —(C₃F₆O)—.
 5. Sulphonic fluorinated ionomers crosslinkedaccording to claim 4, wherein the perfluoropolyether chain R_(f) isselected from the following structures: —(CF₂O)_(a)′(CF₂CF₂O)_(b)′—withb′/a′ in the range 0.1-40, preferably 0.5-20, a′ and b′ being integerssuch as to give the above molecular weight; —(CF₂O)_(c)′(C₃F₆O)_(d)′—with c′/d′ in the range 0.01-5, c′ and d′ being integers such to givethe above molecular weight.
 6. Sulphonic fluorinated ionomerscrosslinked according to claims 4-5, wherein a radical initiatior offormula (f) is used in a concentration in the range 2%-10% by weight,preferably 3-6% by weight with respect to the weight of the sulphonicionomer.
 7. Sulphonic fluorinated ionomers crosslinked according toclaims 1-6, wherein as crosslinking agent the bis-olefin of formula (I)is used in a concentration in the range 3%-25% by weight, preferably4-8% with respect to the weight of the sulphonic ionomer.
 8. Supportedor self-supported membranes obtainable from the sulphonic fluorinatedionomers crosslinked according to claims 1-7.
 9. Supported membranesaccording to claim 8, wherein the support is foamed, preferablybistretched, PTFE.
 10. Supported membranes according to claim 9,obtained by impregnation, preparing a solution formed by: A) a solutionof the sulphonic ionomer in a fluorinated solvent selected from:hexafluoroxylene, perfluorohexane, perfluorooctane, perfluorobenzene,perfluoropolyether solvents, fluoroether solvents. B) a radicalinitiator of formula (d), (e) or (f); C) the bis-olefin of formula (I)as crosslinking agent; a porous support of foamed PTFE being dipped inthe solution formed by A)+B)+C) for a time comprised between 10 secondsand 1 minute and repeating the dipping more times; the impregnatedmembrane being crosslinked at the crosslinking temperature for areaction time equal to 6 half-lives of the used initiator.
 11. Supportedmembranes according to claims 8-9, obtained from solvent by casting. 12.Self-supported membranes according to claim 8, obtained by casting or bypress.
 13. Self-supported membranes according to claim 12, wherein afilm of the solution A)+B)+C) of claim 10 is placed between two metalplates containing a metal frame; the plate/frame/film system being putin a press oven applying a weight of about 500-2,000 kg, preferably750-1,500 Kg.
 14. Membranes according to claims 8-13, wherein thesulphonyl groups —SO₂F are transformed into sulphonic groups —SO₃H by 2steps: salification for transforming the —SO₂F form into the —SO₃K form;acidification for transforming the —SO₃K form into the —SO₃H form. 15.Membranes according to claim 14, wherein the salification is carried outin a basic aqueous solution of KOH or of NaOH at a temperature in therange 5° C.-40° C. for a time comprised between 4 and 40 hours. 16.Membranes according to claims 14-15, wherein the acidification iscarried out in an aqueous solution containing the 20% by weight of HClat 25° C. for 5 hours.
 17. A process for preparing sulphonic fluorinatedionomers crosslinked by radical crosslinking according to claims 1-7.18. A process for preparing supported or self-supported membranesaccording to claims 8-16.
 19. Use of sulphonic fluorinated ionomerscrosslinked according to claims 1-7 for preparing membranes or acidcatalysts.
 20. Use of the membranes according to claims 8-16 in fuelcells or as catalyst.